1. Technical Field
The present disclosure refers to a control device for interleaved converters, to a system of interleaved converters and a related control method.
2. Description of the Related Art
Switch-mode Power Supplies (SMPS) which operate off the public distribution power line, are known as “offline power supplies”. These often comprise switch-mode converters used for the active correction of the input power factor (Power Factor Correctors, PFC). The purpose of a PFC converter is to draw from the power line a current that is essentially sinusoidal and in-phase with the line voltage.
The typical front-end stage of a traditional offline SMPS, made up of a bridge rectifier that receives the ac line voltage at its input and a smoothing/reservoir capacitor connected right across the output of the bridge, produces a non-regulated dc voltage bus with some residual ac component superimposed (ripple). This dc bus will serve as the input for a dc-dc switching converter that will provide one or more regulated output rails to supply the load. These rails are generally isolated from the power line for safety reasons.
The smoothing/reservoir capacitor has large enough capacitance so that this residual ripple is considerably smaller than the dc value, which, therefore, will be only slightly lower than the peak value of the line voltage. As a result, the voltage on the dc side of the bridge is higher than the instantaneous ac voltage applied to the input side most of the time and the bridge rectifiers then conduct only for a small portion of the line cycle. The resulting current drawn from the mains is a series of narrow pulses whose amplitude can be even 5-10 times larger than the resulting average value. To the power line, the SMPS then appears as a strongly non-linear load.
There are considerable drawbacks, as compared to the case of sinusoidal current absorption typical of linear loads: the peak and rms values of the input current will be much higher, the line voltage will be distorted because of the almost contemporary pulsed absorption from all the units connected to the power line, the current into the neutral wire in three-phase power distribution systems will be considerably increased and, on the whole, there will be a poor utilization of the power generation system's capability. In fact, the pulsed current absorbed by offline SMPSs is rich in odd harmonics that do not contribute to the active power transferred to the load and do contribute to increase the current rms value, and it is well known that the power dissipation in wires, and conductors in general, is proportional to the squared rms current.
In quantitative terms, this can be expressed in terms of Power Factor (PF), intended as the ratio of the active power entering the unit (i.e., the power transferred to the load plus that lost inside the unit and dissipated as heat) and the apparent power (the product of the input rms voltage times the input rms current), or of Total Harmonic Distortion (THD), generally intended as percentage ratio of the energy associated to the higher order harmonics to that of the fundamental harmonic. Typically, a traditional SMPS without PFC has a PF around 0.4-0.6 and a THD greater than 100%.
By using switching techniques, a PFC pre-regulator, located between the bridge rectifier and the smoothing/reservoir capacitor, thereby essentially in series to the dc-dc converter that supplies the load, allows the current drawn from the power line to be nearly sinusoidal and in-phase with the line voltage, achieving a PF close to 1 and a low THD (for example lower than 5%), thus addressing the above-mentioned issues.
A PFC pre-regulator is a switch-mode converter controlled in such a way that it generates a regulated dc output bus starting from a rectified ac voltage. In principle, any switching topology (mutual configuration of the main parts constituting the converter) is able to achieve a high PF and a low THD but, in practice, the so-called boost topology, is generally the most advantageous and, then, the most used one.
The boost converter generates an output voltage higher than the input voltage (that is why it is sometimes called “step-up” converter); then, when used in a PFC pre-regulator, it generates an output voltage higher than the maximum line peak voltage, typically set at 400V in systems supplied by the European power line or required to operate world-wide.
There are two major control methodologies, both based on the concept of pulse-width modulation (PWM) currently in use in PFC pre-regulators: the first one, used when the boost converter is operated at fixed frequency with continuous conduction mode (CCM) current of the boost inductor, forces the average boost inductor's current to follow a sinusoidal reference; the second one, used when the boost inductor is operated close to the boundary between CCM and DCM (discontinuous conduction mode), sometimes termed TM (transition mode) operation, forces the envelope of the peak inductor current to follow a sinusoidal reference.
As compared to CCM operation, with the same power level TM operation implies higher peak and rms values for the circulating currents but, on the other hand, it offers soft-switching (i.e., less power loss due to switching) of both the boost diode and the switch without extra components; additionally, the associated control methodology, as well as the practical realization, is simpler. However, the factor that really sets CCM operation apart from TM operation and determines which approach to choose in a given application is the inductor ripple current. To facilitate electromagnetic compatibility (EMC), the high-frequency electrical noise generated by any SMPS and injected back into the power line is kept at a low level. The emission limits are set by relevant EMC regulations and, to comply with them, an LC filter is placed between the input terminals of the SMPS and the bridge rectifier. The high-frequency differential-mode noise that this filter attenuates and keeps below the limits is proportional to the amplitude of the inductor current ripple. For reasons that will be clarified at the end of this section, in a typical TM-operated PFC the amplitude of the current ripple is twice the amplitude of the sinusoidal current drawn from the power line. In a CCM-operated PFC, this ripple will typically be 20-40% of the line current; for the same power level, this means a current ripple 5-10 times smaller. Obviously, the burden for the line filter is enormously alleviated.
This is why TM operation is generally preferred at medium-low power level (typically, <200 W) since the benefits of simplicity and soft-switching often outweigh the demerits of higher RMS currents and greater filtration, while CCM operation is generally suitable to handle higher power levels.
A known PFC pre-regulator in boost topology is shown in FIG. 1.
The boost converter comprises a bridge rectifier 100 having the ac power line at its input (Vin is its rms value); a capacitor Cin, which serves as a high-frequency smoothing filter, connected across the output terminals of a bridge 100, with the negative terminal connected to ground; an inductor L with one end connected to the positive terminal of Cin and including an auxiliary winding; a power switch (typically a MOSFET) M having the drain terminal tied to the other end of the inductor L, the source terminal to ground via a resistor Rs that allows sensing the current flowing through M (i.e., the current flowing through L when M is ON) as a positive voltage drop across Rs itself; a diode D having the anode tied to the node common to L and MOSFET's drain and the cathode connected to the positive plate of the capacitor Co that has its negative plate connected to ground GND. This boost converter generates at its output terminals across Co a dc voltage Vo that will supply the load, typically represented by a dc-dc converter or, in other cases, electronic lamp ballast or a motor.
A control device 10 of the converter comprises an error amplifier VA comparing a portion of the output voltage Vo, brought at its inverting input externally available on pin P1 via the resistor divider R3-R4, with an accurate internal reference Vref connected to the non-inverting input, and generates an error signal proportional to their difference. If the bandwidth of the error amplifier, essentially determined by the frequency compensation network connected between the inverting input and the output (pin P2), is narrow enough—typically below 20 Hz—and a steady-state operation is assumed, the error signal available at pin P2 can be regarded as a dc level, at least as a first approximation.
The error signal is internally fed into one input of the multiplier block 102, which, at the other input, receives a portion of the instantaneous rectified line voltage sensed after the bridge rectifier and brought to pin P3 through the resistor divider R1-R2. The output of the multiplier block will be the product of a rectified sinusoid times a dc level, then still a rectified sinusoid whose amplitude depends on the rms input voltage and the amplitude of the error signal; this will be the reference for PWM modulation.
In fact, the output signal of the multiplier block 102 is fed to the inverting input of a PWM comparator 103 that receives at its non-inverting input the voltage sensed across the sense resistor Rs, which is a voltage proportional to the instantaneous current flowing through the inductor L and the power switch M when this is ON. Assuming M is initially ON, the current through L will be ramping up and so will do the voltage across Rs; when the two input voltages of the comparator are equal, the PWM comparator will reset the SR flip-flop FF and the MOSFET M will be switched off. Therefore, the multiplier output, shaped as a rectified sinusoid, determines the peak value of the inductor and MOSFET currents that, as a result, will be enveloped by a rectified sinusoid.
After the MOSFET M has been switched off the inductor L dumps its energy into the output capacitor Co and the load until it is completely demagnetized. At this point, the diode D opens and the drain node becomes floating; its voltage would tend to eventually reach the instantaneous line voltage through a damped ringing due to its parasitic capacitance that starts resonating with the inductance of L. However, the quick drain voltage fall that follows boost inductor's demagnetization is coupled to pin P5 of the control device through the auxiliary winding in L. The Zero-current Detector (ZCD) block 104 releases a pulse every time there is a negative-going edge on pin P5 and this pulse sets the flip-flop FF and drives the MOSFET ON, hence starting a new switching cycle.
In this way the control drives the beginning of every switching cycle after boost inductor's current has decreased to zero, with a small delay. Because of this delay, often negligible as compared to inductor charging and discharging intervals, the converter will not operate exactly at the DCM/CCM boundary, or in TM, but slightly in DCM; what's more, if the delay is properly chosen, MOSFET's turn-on can be driven when the drain voltage ringing has a valley and, therefore, it is minimized. This condition is termed “valley switching”. It is worth noticing that, if this first pulse generated by the ZCD (Zero Current Detection) block 104 should not switch the MOSFET on for any reason, the drain voltage would keep on ringing; there would be a second oscillation and a second negative-going edge, which would generate a second pulse able to switch on M, and so on, until the oscillation damps to so small values that they can no longer trigger the ZCD block.
In other words, with this kind of operation the converter's switching frequency is “self-synchronized” to boost inductor demagnetization; its value is determined by the time needed to charge and discharge the inductor L and, then, depends on the value of L itself too. Since it is much higher than the line frequency it is possible to express the switching frequency as a function of the instantaneous phase angle θ of the line voltage; neglecting turn-on delay after demagnetization, the relationship is:
            f      sw        =                  1                  2          ⁢                                          ⁢          L                    ⁢              Vin        Iin            ⁢              (                  1          -                                                                      2                                ⁢                Vin                            Vout                        ⁢            sin            ⁢                                                  ⁢            θ                          )              ,where lin is the rms value of the current drawn from the power line. This is an important point for all the considerations that will follow.
In principle, then, a clock/oscillator is not needed to let the converter work. At system's power-up, however, when no signal is available at pin P5, something that initiates the first switching cycle and the subsequent ones is needed, until the signal applied to the ZCD block 104 has an amplitude large enough to be detected by the internal circuits. This is the purpose of the starter block 105, which is disabled once the system gets self-synchronized.
The typical inductor current waveform along a line voltage half-cycle is made up of a series of contiguous (if turn-on delay after demagnetization is neglected) triangles whose height is enveloped by a rectified sinusoid. For apparent geometric reasons, the average inductor current, that is, the cycle-by-cycle average value of the inductor current, will be half the peaks envelope and, then, it will be sinusoidal too. On the other way round, this explains why the inductor current ripple, which here coincides with the total inductor current, is twice the low-frequency line current.
The current drawn from the power line will be the low-frequency component of the inductor current; the switching frequency component, in fact, is almost totally eliminated by the input used for compliance with EMC regulations.
As above mentioned, the TM approach to PFC generally is preferred at lower power levels because of its simpler realization, lower cost and, above all, because of the filtering requirements that make its use impractical at power levels exceeding few hundred watts. It must be said, however, that TM approach has other favorable characteristics.
In a CCM-operated PFC, the transistor M is turned on—and the voltage across D is reversed—before the inductor demagnetizes, that is, while the boost diode D is still conducting current. This produces large current recovery spikes in the boost diode and in the MOSFET, where they cause considerable additional power dissipation; in addition they increase the level of high-frequency electrical common-mode noise generated that needs to be filtered out by the line filter. These issues are completely absent in TM-operated PFC systems, where the MOSFET M is switched on (and the voltage across the boost diode D is reversed) only after the inductor is demagnetized, i.e., when the boost diode current is zero.
In a CCM-operated PFC, when the MOSFET M is turned on, the drain voltage equals the output voltage Vo, and then the power loss associated to the discharge of the drain capacitance inside the MOSFET itself is considerable. Such is also the level of the common-mode electrical noise generated and that is filtered out. In a TM-operated PFC the voltage drain at MOSFET's turn-on is always lower than Vo, even zero under some conditions, and then the associated power loss, as well as the level of common-mode noise, is considerably lower.
In the hundred watts power range, where the worse current form factor of TM operation is not that big a penalty (the rms currents with TM are only 15% larger than with CCM), the efficiency of a TM-operated PFC may be higher than that of a comparable one CCM-operated. On the other hand, in some noise-sensitive applications where PFC is normally used, such as TV or audio equipment, the noise associated to boost diode's reverse recovery and/or drain capacitance discharge may rise serious problems of electromagnetic compatibility inside the piece of equipment itself and make the use of a CCM-operated PFC troublesome.
However, there are practical limits imposed by its heavy filtering requirements, as far as differential-mode noise is concerned.